Tools of the type here considered, designed for drilling, milling, grinding or countersinking, have generally cylindrical working heads with lateral cutting edges on their peripheral surfaces and often, particularly in the case of drill bits, frontal cutting edges on an end face. The lateral cutting edges, which may extend in an axial direction or along helical lines, are separated by flutes codirectional therewith which serve to carry off the chips removed from a workpiece.
In the specific instance of a drill bit, the frontal cutting edges and the corners formed at their junctions with the lateral cutting edges constitute the most severely stressed parts of the tool. It is therefore desirable to make at least these front edges of the hardest and toughest material available. Such materials, developed relatively recently, are ultrahard polycrystalline incisive substances, specially cubic diamond (PCD) and cubic boron nitride (CBN). The use of these materials facilitates machining operations at higher speeds and of better quality than was previously possible, besides lengthening the service life of the tools.
Another advantage, in comparison with monocrystalline cutting materials utilized in tools of earlier vintage, is the greater freedom in the geometrical orientation of the cutting edges since they no longer have to lie in a predetermined position relative to the crystal structure. The polycrystalline materials referred to are suitable, inter alia, for machining workpieces provided with inserts of highly refractory character such as glass-fiber-reinforced conductor plates.
Unfortunately, these ultrahard substances are expensive and are therefore used sparingly in the most highly stressed parts of a tool. Moreover, polycrystalline diamonds have limited thermal stability and for this reason are used mainly with nonferrous metals and synthetic resins whereas workpieces of iron and steel are preferentially machined with the less temperature-sensitive CBN. In the case of drill bits, it is the practice to insert narrow ultrahard blades in pockets on the end faces of the tool bodies, generally made of hard metal, to provide the frontal cutting edges thereof; the blades are soldered to the tool body and are then machined jointly with the latter to form the drill head. Owing to the limited and irregular contact area between these narrow blades and the surrounding metal, the heat generated in a cutting operation is dissipated rather slowly and in a nonuniform manner by the tool body and its mounting so that local thermal stresses can arise. These stresses may initiate or reinforce undesirable vibrations which would impair the quality of the surface being machined. Finally, the handling and positioning of such blades becomes impractical with tools whose end faces have diameters of less than 5 mm.